A typical three-transistor pixel consists of a photodetector, a reset gate, a selection gate and a source-follower transistor. The selection gate and the source-follower transistor are part of a readout circuit, which may include additional components that are shared with other pixels. In use, the photocurrent flowing through the photodetector charges or discharges a capacitance. The voltage stored on the capacitance at the end of a predetermined integration time is detected and represents the intensity of the light falling on the photodetector.
A fundamental problem with conventional imagers results from the fact that the photocurrent is converted to a voltage by integrating the photo-generated charge onto a capacitor. There is a conflict between the desire for a large capacitance to store the photo-generated charge when there is plenty of light and the need for a small capacitance to obtain a high sensitivity in low light conditions.
One method of capturing a HDR image and recording it with a comparatively small number of bits per pixel is to use an image sensor that has a logarithmic output. A high dynamic range pixel having a user-defined response that may provide for example a logarithmic output or another non-linear monotonic output is described in the inventor's international patent application WO20071051964A. This pixel resolves some of the problems associated with earlier pixels. The logarithmic output allows the image sensor to image a scene with a high dynamic range without the pixel output voltage becoming saturated at high photocurrents. The user is able to control the response of the pixel and, as the conversion to a logarithmic value takes place within the pixel, it is very quick and does not significantly increase the cost of the pixel.
However, one limitation of this pixel is that at very low photocurrents it may be unable to provide a logarithmic response. This is because operation of the pixel depends on reducing the effective integration time and at very low photocurrents the pixel may not be discharged sufficiently within the maximum integration time to create a logarithmic response.
It is therefore an object of the present invention to provide a high dynamic range pixel that mitigates one or more of the aforesaid disadvantages.